Square dumbbell



FIG. 1 is a front, top and right side isometric view, of a square dumbbell, showing my new design, the stippling on the surfaces is to show the surface character of the item, and forms a part of the claimed design;

FIG. 2 is a left side elevational view thereof; and,

FIG. 3 is a rear elevational view thereof, the bottom view being identical to the top as shown in FIG. 1. 

The ornamental design for a square dumbbell, as shown and described. 